There are many known rollers or wheels designed for supporting a load. For example, U.S. Pat. No. 1,408,388 to Reddaway et al. is directed to a tire and rim assembly wherein the tire includes a body portion formed of hard rubber and a central portion constituting a soft rubber resilient core. U.S. Pat. No. 1,687,113 to Stockdale describes a caster wheel formed with a rubber annulus and a tread or wear element in the shape of a disc which is fabricated of fibrous or fabric materials vulcanized to the rubber annulus. U.S. Pat. No. 4,321,727 to Sheiman et al. discloses a luggage roller including a central groove in the form of a V-shaped channel of elastomeric material and a central cylindrical sleeve formed of low friction material, such as polyethylene nylon, Teflon, or similar plastic. In U.S. Pat. No. 4,452,141 to Mistyurik, a roller with a porous surface and a central flange is disclosed. U.S. Pat. No. 5,803,235 to McGinnis et al. is directed to an idler roller which includes an impact shell with a backing layer on the inside of the shell, and a compression ring adapted to expand and engage the inside of the impact shell and an external surface of a main roller body. The load carrying capability of these types of rollers and other rollers varies as a result of their structural construction and intended use.
Blasthole drills are large machines used in mining operations to drill holes for explosives. A conventional blasthole drill comprises a frame supported by crawlers for movement over the ground. A mast which defines a drill hole axis is supported by the frame for movement between a substantially vertical position and a number of angled or non-vertical positions. A rotary head, which engages the upper end of a drill pipe for rotating the drill pipe and driving the drill pipe into the ground, moves relative to the mast along the drill hole axis. The rotary head is supported by rollers which move along appropriate surfaces or grooves in the mast. The rollers are designed to have a load carrying capacity to support, at a minimum, the weight of the rotary head and the weight of the end of the drill pipe supported by the rotary head. As can be appreciated, when the mast is positioned in a vertical or substantially vertical position, a light load is transferred to the rollers. On the other hand, when the rotary head is applying, a downward force for drinking, heavy load is transferred to the rollers.
It is known to make such rollers of an all-steel construction.
It is also known to provide rollers having a steel main body surrounded with an outer covering of plastic.